Polymer containing dispersions of photographic couplers have been employed in photographic materials, as described, e.g., in U.S. Pat. Nos. 4,857,449; 5,001,045; 5,047,314; 5,055,386; 5,200,304; 5,242,788; 5,294,527. A main advantage of polymer-containing dispersions described in the prior art relates to image preservability to heat and light, although other advantages in manufacturing processes, physical performance of the photographic element, and sensitometric performance have been reported. Methods for preparing polymer-containing dispersions of dye-forming couplers are described in, e.g., U.S. Pat. Nos. 4,939,077; 4,203,716; and 4,840,885. Commonly, these dispersions are prepared from a solution of a coupler, an optional high-boiling solvent, an oil-soluble but water-insoluble polymer, and a volatile organic solvent, which solution is then emulsified and dispersed in an aqueous solution, often comprising water, a hydrophilic colloid such as gelatin, and a surfactant. Other methods describe the formation of loaded latex polymer dispersions using water-miscible or volatile organic solvent.
Preferred polymer compositions reported in the prior art have generally focused on relatively high glass transition temperature (T.sub.g) alkylacrylamide polymers such as poly(t-butylacrylamide) and other high T.sub.g polymers such as styrene/methylmethacrylate copolymers. Poly(t-butylacrylamide) has a T.sub.g near 146.degree. C. in the dry state, and styrene/methylmethacrylate copolymers will have T.sub.g 's greater than about 1000.degree. C. Such prior disclosures typically indicate polymers having T.sub.g greater than 60.degree. C. are preferred, and more preferably greater than 90.degree. C., for light stability of formed image dyes.
Despite the benefit of light stability provided by polymers such as poly(t-butylacrylamide), several concerns have been raised with the use of this technology. Elements with the high T.sub.g polymer-containing coupler dispersions, e.g., may be more pressure sensitive compared to conventional solvent coupler dispersions. Additionally, conditions of heat and humidity can cause smearing of the formed image dye in a final print, leading to density increases that show up visually as a color shift. This phenomenon, referred to herein as TIC (Thermally Induced Change), may occur in hot, humid oven-keeping tests of photographic elements over several hours or days, or it can occur very quickly in overly harsh drymount press conditions.
Previous work with pressure-sensitive emulsion layers in color negative materials had shown that low T.sub.g materials coated adjacent to or in the pressure sensitive layer could decrease the signal (see, e.g., U.S. Pat. Nos. 5,015,566; 5,066,572; 5,300,417; 5,310,639). Use of many relatively low T.sub.g polymers such as polybutylacrylate, however, has been found to be much less effective than use of the high T.sub.g poly(t-butylacrylamide) at stabilizing dyes to light fade, and that at the higher levels of polymer needed for image stability, TIC dye smear is generally more severe with the lower T.sub.g polymers.
It may also be desirable to provide even more light stability beyond what is practically achievable with the high T.sub.g polymers preferred in the prior art. In many polymer/coupler systems, e.g., while additional polymer may improve light stability, it is at the impractical expense of much worsened pressure sensitivity and/or TIC. A related set of trade-offs exists with variation of polymer latex particle size. Smaller sized latex particles offer improved image light stability, but at the expense of much worsened TIC and perhaps somewhat worsened pressure sensitivity. In the face of these direct trade-offs, no formulation options have been previously proposed to simultaneously improve all three properties.